This invention relates generally to analyzing imbalance of a rotating vehicle wheel and in particular to determine the magnitude and angular location of corrective counter-balance mass(es) to be placed on the wheel rim(s).
The determination of unbalance in vehicle wheels is carried out by an analysis with reference to phase and amplitude of the mechanical vibrations caused by rotating unbalanced masses in the wheel. The mechanical vibrations are measured as motions, forces, or pressures by means of transducers, which convert the mechanical vibrations to electrical signals. Each signal is the combination of fundamental oscillations caused by the imbalances, and noise.
It is well known in the art that a variety of types of correction weights are available for placing on the wheel to correct the measured imbalance. For example, both adhesive-backed weights and hammer-on weights are available from a number of different manufacturers. These weights come in discrete units, such as 1/4 ounce, 1/2 ounce, 1 ounce, etc. which do not always correspond to the desired corrective weight magnitude. Many systems have been designed to compensate in some way for the limitations of the correction weights available.
For instance, U.S. Pat. No. 4,891,981 to Schonfeld calculates a different angle than the optimal angle for each plane in order to use the exact incremental size weights. A larger or smaller weight circle is tried for each plane (by the CPU). The best of four calculated combinations is displayed to exactly cancel static and reduce to an "accepted" minimum dynamic residual unbalance.
U.S. Pat. No. 4,759,217 to Brihier et al. discloses a calibration process of measuring with two weights on two planes at an angle, then moving them and re-measuring, allegedly without any need to know width, diameter, or reference distance.
U.S. Pat. No. 4,348,885 to Mueller describes a calibration method which uses an add-on clamping fixture with known geometry and mass locations to eliminate the need to enter parameters.
U.S. Pat. No. 4,193,304 to Hofmann concerns a system for wheel balancing which provides a "correction value" step to account for the center of mass location of the correction weight. That is, it appears to compensate for the difference between the nominal value of the balancer's weight circle and the true weight circle.
U.S. Pat. No. 4,068,532 to Green et al. relates to a wheel balancer which uses a single size weight for balancing.
U.S. Pat. No. 3,890,845 to Muller describes a balancing system with 120/60 degree placement of test weights and the removal of material for balancing rotors.
U.S. Pat. No. 3,550,455 to Green et al. is directed to a single weight value system of fanning the weights on a gravity system.
And U.S. Pat. No. 3,251,230 to Green et al. relates to a slidable test weight.
None of these prior patents mention anything about weight curvature and how that curvature affects the effective weight of the corrective weight.
A system which addresses some of these concerns is shown in co-assigned U.S. patent application Ser. No. 07/824,999, filed Jan. 24, 1992, the disclosure of which is incorporated herein by reference. The system of said patent application solved many of the problems caused by the discrete nature of the available correction weights, but even that system can be improved.
For example, it has been found that larger weights, whether adhesive or hammer-on, have an effective weight (the weight actually measured by the force pickups) different than the actual or applied weight. This effect is caused by the curvature of the weight, which becomes pronounced for larger weights. Curvature of the weight shifts the center of gravity (cg) of some weight towards the center of the wheel. Take a three oz. weight which actually does weigh precisely three oz. When that three oz. weight is placed on a 14" wheel which has previously been balanced to zero, the wheel balancer will show (for example) 2.92 oz. imbalance due to the application of that weight instead of the three oz. which was actually applied. Furthermore, the 2.92 oz. imbalance cannot be corrected by applying another weight, trimmed to 2.92 oz., because the 2.92 oz. weight has an effective weight of 2.82 oz. or so.
The problem is more severe the larger the imbalance. A 3.75 oz. weight will read 3.60. If the machine (the balancing apparatus) has a rounding mode enabled (which, for example, rounds to the nearest 0.25 oz.), the correction weight will be shown as 3.5 oz. Similarly, if the actual imbalance is 3.6 and the machine rounds the display to 3.5 oz., the user will apply a 3.5 oz. weight. Unfortunately, because of the center of gravity/curvature effect, the effective weight of that 3.5 oz. weight is about 3.38 oz. The user should have applied a 3.75 oz. weight. There will be almost 0.22 oz. residual imbalance (3.6-3.38).
The smaller the diameter rim, the more severe is this effect because of the greater curvature (the weights bend quite easily to conform to the rim).
Unfortunately, none of the patents listed above or the patent application appear to address this center of gravity/curvature problem which occurs with larger weights and smaller diameter wheels.